Introducing different flow direction algorithms to map topographic wetness index and soil organic carbon in a loess hillslope of Toshan area, Golestan Province, Iran

Topography is one of the major factors controlling the spatial pattern of saturated areas, which in turn is a key to understand much of the variability in soils and hydrological processes. The topographic wetness index (TWI) which combines local upslope contributing area and slope, is commonly used to quantify topographic control on hydrological processes. On the other hand, the interaction of topography with the climate affects the distribution of soil organic matter (SOM) as one of the most effective soil quality parameters that has been shown to correlate with TWI and can be predicted by TWI. The main objective of this study was to evaluate different flow direction algorithms of TWI and to select suitable algorithm for the study area and to map soil organic carbon in a loess hillslope of Toshan area, Golestan Province, Iran. TWI was calculated by single flow direction (SFD), biflow direction (BFD) and multiple flow direction algorithms (MFD). 135 Samples were collected at random systematic method (0-20 cm depth). The results showed D-Infinity (BFD) and MFD algorithms in the study area had stronger correlation with SOM than SFD (D8 and RHO8) algorithms. However TWIs used in most studies are calculated using a SFD algorithm, which assumes that all water from a grid cell flows into only one neighboring cell. But this assumption is not always valid, especially in areas with low relief where movement of water may be divergent. Finally, TWI is suggested to be calculated with different algorithms so that the best method could be selected to obtain accurate results.